Oil sand is essentially a matrix of bitumen, mineral matter and water. The bitumen component of oil sand consists of viscous hydrocarbons which behave much like a solid at normal in situ temperatures and which act as a binder for the other components of the oil sand matrix. The mineral matter component of oil sand typically consists largely of sand, but may also include rock, silt and clay. Sand and rock are considered to be coarse mineral matter, while clay and silt are considered to be fine mineral matter, where fines are defined as mineral matter having a particle size of less than 44 microns. The water component of oil sand consists essentially of a film of connate water surrounding the sand in the oil sand matrix, and may also contain particles of fine mineral matter within it.
A typical deposit of oil sand will contain about 10% to 12% bitumen and about 3% to 6% water, with the remainder of the oil sand being made up of solid mineral matter particles. Typically the mineral matter component in oil sand will contain about 14% to 20% fines, measured by weight of total mineral matter contained in the deposit, but the amount of fines may increase to about 30% or more for poorer quality deposits. Oil sand extracted from the Athabasca area near Fort McMurray, Alberta, Canada, averages about 11% bitumen, 5% water and 84% mineral matter, with about 15% to 20% of the mineral matter being made up of fines.
Oil sand deposits are mined for the purpose of extracting bitumen from the oil sand, which bitumen is then upgraded to synthetic crude oil. Accordingly, various processes have been developed for extracting the bitumen from the oil sand.
For instance, conventionally, a “hot water process” is used for extracting bitumen from oil sand in which both aggressive thermal action and aggressive mechanical action are used to liberate and separate bitumen from the oil sand. The hot water process is a three step process. First, the oil sand is conditioned by mixing it with hot water at about 95° Celsius and steam in a conditioning vessel which vigorously agitates the resulting slurry in order to completely disintegrate the oil sand. Second, once the disintegration is complete, the slurry is separated by allowing the sand and rock to settle out, and the bitumen, having air entrained within it, floats to the top of the slurry and is withdrawn as a bitumen froth. Third, the remainder of the slurry, which is referred to as the middlings, is then treated further or scavenged by froth flotation techniques to recover bitumen that did not float to the top of the slurry during the separation step.
To assist in the recovery of bitumen during the separation step, sodium hydroxide (caustic) is typically added to the slurry during the conditioning step in order to maintain the pH of the slurry slightly basic, in the range of 8.0 to 8.5. This has the effect of chemically dispersing the clay that becomes dispersed in the slurry during the conditioning step, which in turn reduces the viscosity of the slurry by reducing the particle size of the clay minerals present in the slurry. With the clay present in the slurry chemically dispersed and the viscosity of the slurry lowered, the bitumen more readily floats to the surface of the slurry and can therefore be more readily recovered during the separation step.
There are several disadvantages to the hot water process. The use of hot water and steam in the process, as well as the vigorous agitation to which the oil sand is subjected during the conditioning step, mean that the energy requirements of the process are very high. In addition, since the main goal of the hot water process is to liberate and separate bitumen from the oil sand by completely destroying the oil sand matrix, most of the fine mineral matter contained in the oil sand becomes mechanically dispersed throughout the slurry during the conditioning step.
The addition of caustic to the slurry to reduce the viscosity of the slurry results in further chemical dispersal of the clay in the fine mineral matter, whereby the size of the individual clay particles may be reduced to as small as 0.2 microns. The combination of the vigorous and complete physical dispersal of the fines contained in the oil sand and the chemical dispersal of the clay in the resulting slurry create a middlings stream that may contain a large amount of very well dispersed fines held in suspension, particularly where the oil sand deposit is of lower quality and therefore has a relatively high fines content. As the fines content of the oil sand feedstock increases, the concentration of fines in the slurry increases, and recovery of bitumen from the slurry becomes more difficult, since the suspended fine particles tend to “trap” bitumen within the slurry.
In addition to the problems regarding the recovery of bitumen from slurries containing a large amount of dispersed fines, the middlings stream that remains following the scavenging step poses a huge disposal problem, since it constitutes a sludge that tends to settle and consolidate very slowly. Typically, the practice for the disposal of the sludge remaining after the scavenging step involves pumping it into huge tailing ponds, where the fines slowly settle and stratify. After several weeks, some of the water forming the sludge will be present at the top of the tailing pond containing only a small amount of suspended fines. This water may be recycled for use in the hot water process, after being reheated to the process temperature.
In any event, because of the characteristics of the middlings sludge, the tailing ponds cannot be completely rehabilitated for many, many years, and only a portion of the water that enters the tailing ponds can be recovered and reused in the hot water process, thus creating a requirement that a large amount of makeup water be available for the hot water process to make up for the water that is lost to the tailing ponds.
Some attempts have been made to improve upon the hot water process, such as: Canadian Patent No. 1,085,761 issued on Sep. 16, 1980 to Rendall; U.S. Pat. No. 4,512,956 issued on Apr. 23, 1985 to Robinson et al; U.S. Pat. No. 4,533,459 issued on Aug. 6, 1985 to Dente et al; U.S. Pat. No. 4,414,117 issued on Nov. 8, 1983 to Yong et al; and U.S. Pat. No. 4,225,433 issued Sep. 30, 1980 to Liu et al. However, none of these attempts have been found to be fully satisfactory.
The challenge remains to extract bitumen from oil sand in a manner maximizing the recovery of bitumen while minimizing the amount of sludge that is generated, and while controlling the physical characteristics of the sludge so that it may be more easily disposed of. It is also desirable to minimize the energy requirements of the process as much as possible so that the process can be carried out in an economical and environmentally acceptable manner.
In this regard, Canadian Patent Application No. 2,030,934 published on May 28, 1992 by Strand and Canadian Patent Application No. 2,124,199 published on Jun. 11, 1992 by Strand, both describe an extraction apparatus and process employing a countercurrent separator vessel in which oil sand is gently rolled from one end to the other by a spiral ribbon and mixer elements while hot water, defined as having a temperature of 50° Celsius, circulates in the opposite direction. Two streams are then removed from opposite ends of the separator vessel. One stream contains coarse material and some water, while the other stream contains bitumen and dispersed fines in a slurry. Mechanical action is minimized and liberation and separation of bitumen is accomplished almost entirely by thermal action.
It is stated in these applications that an important objective of the invention is to leave most of the clay in the oil sand in its original state so that it may be returned along with separated coarse material, to the site from which the oil sand was mined. It is also stated that due to limited dispersal of clay in the process water, it should not normally be necessary to add caustic to aid in the recovery of bitumen, and a substantial portion of the process water will be available for recycling. As for the amount of process water required, it is stated that the water to oil sand ratio is a function of the heat transfer requirements of the system, and not the requirement to provide adequate dilution of the slurry to facilitate bitumen recovery.
Further, Canadian Patent No. 2,123,076 issued Nov. 17, 1998 to Strand et. al. utilizes the countercurrent separator vessel of the previously noted Canadian Patent Applications in the performance of an improved oil sand extraction process. Specifically, Strand et. al. describes an overall method for processing lumps of oil sand containing bitumen to produce a bitumen froth and non segregating tailings of a solid material and a sludge.
The method includes depositing the lumps of oil sand into a bath of warm water. The lumps are then conditioned by gently contacting them with the warm water to liberate and separate bitumen from the oil sand while minimizing the dispersal into the bath of fine material contained in the oil sand. The conditioning step is preferably performed utilizing the previously described countercurrent separator vessel, as shown in FIGS. 2 and 3 of Canadian Patent No. 2,123,076.
Following conditioning, the solid material remaining after the liberation and separation of the bitumen from the oil sand is removed from the bath and collected for further processing. The warm water containing bitumen and dispersed fine material is also removed from the bath and collected for further processing.
Following removal from the bath, the warm water containing bitumen and dispersed fine material is separated into the bitumen froth and a suspension of dispersed fine material. The suspension of dispersed fine material is dewatered to produce the sludge, which is combined with the solid material to produce the tailings. Preferably, the sludge is combined with the solid material in a mixing drum as shown in FIG. 4 of Canadian Patent No. 2,123,076.
The stated goal of Canadian Patent No. 2,123,076 is to eliminate or reduce the need for sludge tailing ponds which typically occupy many square kilometers, and replace the sludge currently disposed of in these tailing ponds with nonsegregating tailings produced from both the solid material generated by the extraction process and the sludge generated by the extraction process. In order to minimize the energy requirements of the described process, the thermal and mechanical energy input into the process are limited, while also limiting the amount of thermal energy that is lost during the process to the various product and waste streams.
However, there continues to be a need for improvements to be made to the oil sand processing methods and apparatuses in order to increase the efficiencies and to enhance or improve upon the characteristics or qualities of the resulting products of such methods and apparatuses.
Accordingly, there is a need in the industry for an improved apparatus for processing oil sand to produce a liquid stream and a solids stream having desirable characteristics or qualities. Further, to enhance or facilitate the efficient operation of the improved apparatus, there is a need for an improved control system and method for controlling the apparatus.